Line pipes used for transporting natural gas and crude oil have recently been increased in strength every year in order to improve transportation efficiency by increasing pressure and improve field welding efficiency by decreasing thickness. Also, there have been put into practical use line pipes having high deformability (representing that large uniform elongation occurs under external stress to prevent buckling, and elongation has allowance because of a low yield ratio), i.e., a tensile strength of over 800 MPa, in order to prevent crack initiation due to local buckling even when large deformation occurs in line pipes by large earthquake or ground movement in a permafrost region. In recent years, the requirement for line pipes to have a tensile strength of over 900 MPa has been being realized.
With respect to a method of producing a steel plate for welded steel pipes for such high-strength line pipes, for example, Patent Document 1 discloses a technique in which two-step cooling is performed after hot-rolling, and the cooling stop temperature in the second step is 300° C. or less for achieving high strength.
Patent Document 2 discloses a technique relating conditions for accelerated cooling and aging heat treatment for increasing strength by Cu precipitation strengthening. Further, Patent Document 3 discloses a steel pipe having excellent resistance to buckling against compression and having an appropriate area fraction of a second phase structure according to the ratio of the pipe thickness to the external diameter, thereby exhibiting a low yield ratio.
However, like in the technique disclosed in Patent Document 1, when the cooling stop temperature is decreased to introduce a hard bainite or martensite structure which produces low-temperature transformation, thereby achieving high strength, a crack (referred to as a “cutting crack” hereinafter) occurs in a cut end surface due to diffusible hydrogen remaining in steel when the cooled steel plate is cut into a necessary size by shearing. There is demand for a steel plate having a tensile strength of less than 900 MPa to have high deformability. However, a steel plate having a yield ratio of 0.85 or less has not yet been obtained.
On the other hand, like in Patent Document 2, when heat treatment is performed after accelerated cooling, hydrogen in steel is sufficiently diffused, and thus the occurrence of a cutting crack can be suppressed. However, cementite is precipitated and coarsened in the microstructure during the heat treatment, thereby decreasing toughness and particularly degrading DWTT (Drop Weight Tear Test) properties for evaluating brittle crack arrestability. Patent Document 2 is not aimed at high deformability, and thus a yield ratio of 0.85 or less is not achieved.
Further, as described in Patent Document 3, the technique disclosed in this document is aimed at decreasing a yield ratio (YR) obtained by dividing yield strength by tensile strength in order to comply with the requirement for high deformability for preventing the occurrence of cracks even when large deformation is produced in a line pipe by large earthquake or ground movement in a permafrost region. However, in this technique, the microstructure of steel pipe is dual phase, and thus Charpy absorbed energy is decreased. Therefore, the crack arrestability of ductile fracture caused by exogenous trouble is not excellent (A brittle fracture test is performed by applying a static or dynamic load to a test piece or specimen provided with a notch or subjected to processing alternative to notching. In this test, a brittle crack is produced by impact load, and the brittle fracture arrestability is determined at each temperature.), and a tensile strength of 900 MPa or more cannot be achieved because a first phase has a ferrite structure.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-293089
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 08-311548
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 09-184015